NaCl (100 mM) reduced the potency of (+)-N-methyl-4-methyldiphenhydramine ((+)-QMDP) as an inhibitor of the binding of [3H]-mepyramine to histamine H1-receptors on guinea-pig cerebellar membranes to a greater extent than that of mepyramine, consistent with the greater inhibitory effect of Na+ on the binding of [3H]-QMDP than on the binding of [3H]-mepyramine. 2. The concentration of 2-amino-2-hydroxymethyl-propan-1,3-diol HCl (Tris, HCl) buffer, pH 7.5, present had little effect on the temelastine-insensitive binding of [3H]-mepyramine, but caused a concentration-dependent inhibition of the binding of [3H]-mepyramine sensitive to 1 microM temelastine (H1-receptor binding), with an approximate IC50 of 75 mM, assuming that complete inhibition would have been achieved. 3. Inhibition of [3H]-mepyramine binding by Na+ was more marked in 10 mM than in 50 mM Tris HCl and was not evident in 200 mM Tris HCl. 4. The Kd for the temelastine-sensitive binding of [3H]-mepyramine measured in 10 mM Tris HCl, 0.24 +/- 0.01 nM, was increased by 2.2 +/- 0.2 fold by 100 mM NaCl, without any significant change in the maximum binding (Bmax). The Bmax for [3H]-mepyramine was similarly unchanged in 50 mM Tris HCl, but the Kd was increased 2.5 +/- 0.2 fold. 5. The Kd for the temelastine-sensitive binding of [3H]-mepyramine was also increased in 50 mM,compared with 10 mM, N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulphonic acid] KOH (HEPES.KOH)buffer (Kd 0.25 +/- 0.02 nm in 10 mM HEPES), but the evidence for an interaction between HEPES and Na+ was less clear.6. The effect of 100 mM NaCl on the inhibition of [3H]-mepyramine binding in 10 mM Tris HCl was examined for a range of antagonists. The decrease in potency caused by Na+ was greatest for triprolidine, (+)-chlorpheniramine and benzilylcholine (9.6-10.3 fold increase in K1 values) but the binding of mepyramine and promethazine was much less affected (1.8 and 1.9 fold increase in Kd respectively). The Kd for temelastine was not significantly changed. In contrast to the general decrease in antagonist affinity in the presence of Na+, the for MDL 16,455A (4-[1-hydroxy-4-[4-(hydroxydiphenylmethyl)-1-piperidinyl]butyl]-alpha,alpha-dimethylbenzene acetic acid) was increased, but only by 1.5 fold.7. It is concluded that Na+ can act as an allosteric effector of the binding of antagonists at the histamine HI-receptor. Tris HCl also appears to have an allosteric action at the H1-receptor.
摘要
氯化钠（100 mM）使（+）-N-甲基-4-甲基苯海拉明（（+）-QMDP）作为[3H]-美吡拉敏与豚鼠小脑膜上组胺H1受体结合抑制剂的效力降低的程度，比美吡拉敏更大，这与钠离子对[3H]-QMDP结合的抑制作用比对[3H]-美吡拉敏结合的抑制作用更强一致。2. 存在的pH 7.5的2-氨基-2-羟甲基丙烷-1,3-二醇盐酸盐（Tris，HCl）缓冲液浓度，对[3H]-美吡拉敏的替美斯汀不敏感结合影响很小，但对1 microM替美斯汀敏感的[3H]-美吡拉敏结合（H1受体结合）产生浓度依赖性抑制，假设完全抑制能够实现，其近似IC50为75 mM。3. 钠离子对[3H]-美吡拉敏结合的抑制在10 mM Tris HCl中比在50 mM Tris HCl中更明显，而在200 mM Tris HCl中不明显。4. 在10 mM Tris HCl中测得的[3H]-美吡拉敏的替美斯汀敏感结合的Kd为0.24±0.01 nM，100 mM氯化钠使其增加2.2±0.2倍，最大结合量（Bmax）无显著变化。在50 mM Tris HCl中[3H]-美吡拉敏的Bmax同样未改变，但Kd增加2.5±0.2倍。5. 与10 mM相比，在50 mM N-[2-羟乙基]哌嗪-N'-[2-乙磺酸]氢氧化钾（HEPES.KOH）缓冲液中，[3H]-美吡拉敏的替美斯汀敏感结合的Kd也增加（在10 mM HEPES中Kd为0.25±0.02 nM），但HEPES与钠离子之间相互作用的证据不太明确。6. 针对一系列拮抗剂，研究了100 mM氯化钠对10 mM Tris HCl中[3H]-美吡拉敏结合抑制的影响。钠离子导致效力降低对曲普利啶、（+）-氯苯那敏和苄基胆碱最大（K1值增加9.6 - 10.3倍），但美吡拉敏和异丙嗪的结合受影响小得多（Kd分别增加1.8倍和1.9倍）。替美斯汀的Kd无显著变化。与存在钠离子时拮抗剂亲和力普遍降低相反，MDL 16,455A（4-[1-羟基-4-[4-（羟基二苯甲基）-1-哌啶基]丁基]-α,α-二甲基苯乙酸）的亲和力增加，但仅增加1.5倍。7. 得出结论：钠离子可作为组胺H1受体拮抗剂结合的变构效应剂。Tris HCl似乎在H1受体处也有变构作用。
